1. Field of Invention
This invention relates to dental restorative composite compositions, and particularly, to the use of titanium silicate as the finely divided, inorganic filler in dental restorative composites.
2. Discussion of the Prior Art
Dental restorative composites, generally in the form of highly filled blends of a liquid polymerizable organic resin matrix and finely divided inorganic particulate filler, have achieved wide commercial success and are used extensively in clinical dental practice. Basically, most of the dental restorative composites which have become commercially available or which are described in the literature, are based upon the development of the system first disclosed by Bowen in U.S. Pat. No. 3,066,112.
In the direct filling system or restorative composite described by Bowen in this patent, the liquid polymerizable organic resin matrix or binder is principally the reaction product of bisphenol A and glycidyl methacrylate, referred to as BIS-GMA, preferably combined with one or more other active monomers, referred to as reactive diluents, especially other dimethacrylates, for example, triethylene glycol dimethacrylate. The system also includes a catalyst or polymerization initiator, such as, for example, benzoyl peroxide and preferably, also, to allow the polymerization to take place in a reasonable period of time, a polymerization accelerator or activator, such as, for example, N, N-dimethyl-p-toluidine. A particularly attractive combination of catalyst and accelerator is a hydroperoxide, more stable than benzoyl peroxide, and a substituted thiourea, less prone to cause coloration than an amine accelerator, as disclosed in U.S. Pat. No. 3,991,008. Other ingredients, such as stabilizers or UV-absorbents may also be present in association with the polymerizable constituents to increase shelf life and otherwise prevent degradation of the properties of the restorative composite composition. Still further, these restorative composite compositions may include various dyes or pigments to obtain various shades to conform to the color of the tooth structure with which the restorative composite material is being used.
The composite restorative materials are generally provided for commercial use as multi-package systems, most typically a two package system with the system described in U.S. Pat. No. 3,926,906 to Lee et al. being most typical. In these systems, the reactive monomers are generally provided in the form of a paste blended with the finely divided inert inorganic filler with the reactive diluent and/or catalyst and/or activator maintained separately from the polymerizable ingredients or reactive diluent.
The most commonly used inorganic filler materials are typically crystalline quartz or amorphous silica, although other materials, such as, for example, fused silica, crystalline silica, glass, fused alumina and the like, have also been disclosed. It is also common practice to treat the filler or the binder or both, with a coupling agent, such as .gamma.-methacryloxypropyltrimethoxysilane, to enhance adhesion between the organic matrix binder and the inert inorganic filler particles.
Other fillers have also been suggested which have a negative coefficient of thermal expansion. For instance, U.S. Pat. No. 3,503,128 suggests the use of .beta.-eucryptite, a lithium aluminum silicate. The use of fillers of low or negative coefficients of thermal expansion is highly desireable in order to more closely match the resultant composite with the tooth structure in terms of thermal expansion. In general, composite restorations which are highly loaded with the inorganic filler particles are more nearly compatible with tooth structure than the previously used unfilled resin restorative compositions. Thus, over the pertinent region of 0-60.degree. C., tooth enamel has a coefficient of thermal expansion of about 11.times.10.sup.-6 (or 11 ppm), whereas, an unfilled resin has a corresponding value of coefficient of thermal expansion of about 80-100 ppm.
However, restorative composites based on .beta.-eucryptite and other fillers with negative or close to zero coefficients of thermal expansion are characterized by poor physical properties and particularly, low compressive strength.